The use of selective adsorbent materials in the enrichment of gas mixtures with desired components thereof is well known. For example to derive an oxygen-rich gas from an air feedstock it is known, as taught e.g. by U.S. Pat. No. 3,923,477 of J. W. Armond et al, to pass the air through a bed of material which preferentially adsorbs nitrogen, such as a zeolite molecular sieve, and to collect the unadsorbed components issuing from the bed as the desired product. In practice a cyclic operation is employed in which each cycle comprises the successive steps of passing the feedstock through the bed and collecting the product, regenerating the bed by evacuating or otherwise desorbing the adsorbed components from the bed, and refilling the bed in preparation for the next cycle. Materials which preferentially adsorb oxygen, such as molecular sieve carbon, can also by employed in a similar process to derive an oxygen-rich gas from an air feedstock, in this case the adsorbed components being collected from the bed as the desired product during the regeneration step of each cycle.
Although the pressure swing adsorption (psa) processes described above offer a relatively simple and inexpensive route to the production of oxygen-rich gas streams from air, as compared with conventional cryogenic air separation techniques, their usefulness in practice is limited by their inability to provide a product of sufficiently high purity for many applications. For the purposes of this specification a `high purity` product is to be regarded as one in which the oxygen content is in the order of 99% or more.
Various refinements to the basic processes described above can be made with a view to increasing the purity of the product, but even so the maximum product purity which can be attained with the zeolite molecular sieve process is no more than 95% oxygen, and with the molecular sieve carbon process in only about 80% oxygen.
This can be explained as follows. The major constituents of atmospheric air are, in order, nitrogen, oxygen, argon and carbon dioxide, plus a varying proportion of moisture (water vapour). Present-day synthetic zeolite molecular sieves as used in a psa process as described above have the ability to effect virtually complete separation as between oxygen and nitrogen. Similarly moisture and carbon dioxide can be effectively separated off during passage of the feedstock through a zeolite bed. However, the adsorption isotherms of oxygen and argon on these materials are almost identical so that passage through a zeolite bed has no significant effect on the ratio of oxygen to argon in a gas mixture. Thus with an air feedstock the ratio of oxygen to argon in the product will inevitably be approximately the same as the atmospheric ratio of these gases, with the result that however effectively the oxygen is separated from the other major constituents of the feedstock there will still be a balance of about 5% argon in the oxygen product.
As regards the molecular sieve carbon process, the selectivity of adsorption exhibited by this material is the result of a rate effect in comparison with the zeolite sieve whose selectivity is the result of a capacity effect. In other words, whereas a separation of oxygen from nitrogen is achieved with a zeolite sieve by virtue of the material's ability to hold nitrogen more strongly than oxygen, the separation achieved with the carbon sieve is as a result of the material's more rapid adsorption of oxygen than of nitrogen. From the point of view of oxygen/nitrogen separation the rate effect of the carbon sieve is significantly less efficient than the capacity effect of the zeolite sieve and the oxygen product obtainable from an air feedstock with the carbon sieve process inevitably contains a considerable proportion of unseparated nitrogen. In practice the rates of adsorption of nitrogen and argon on molecular sieve carbon are about the same so that with an air feedstock the balance of the oxygen product will contain nitrogen and argon approximately in their atmospheric ratio. Carbon dioxide and moisture are adsorbed at a greater rate than nitrogen and argon so that unless the air is pre-treated to remove these consituents they will also be present as contaminants of the oxygen product.
To restate the position then, the zeolite sieve process can give a maximum oxygen purity of about 95% with the balance represented virtually entirely by argon, and the carbon sieve process, assuming pretreatment to effectively remove carbon dioxide and moisture, can give a maximum oxygen purity of about 80% with the balance represented by nitrogen and argon in their atmospheric ratio, i.e. about 19.75% nitrogen and 0.25% argon. For these reasons usage on a commercial scale of oxygen produced by psa air separation has hitherto been restricted to certain applications where high purity is not essential, notably in the treatment of sewage, furnace atmosphere enrichment and chemical oxidation processes. More general acceptance of psa as a commercially valuable means of producing oxygen from air is unlikely to be achieved until the art has progressed to the point where the product can regularly be obtained at a purity significantly better than that achieved hitherto and such as is conventionally obtained by cryogenic air separation techniques. By way of example, the efficiency of welding and cutting processes using oxygen is greatly dependent upon the purity of the gas available, and for these applications a purity of at least 99.5% is customarily specified. Likewise hospital and health authorites are most reluctant to accept anything less than a purity of this order for medical applications.
Accordingly it is a particular object of the present invention to provided a psa air separation process which will be capable of providing an oxygen product at the high purity demanded by most consumers and underlying the invention is the realisation that by suitably integrating the zeolite and carbon sieve processes described above an oxygen product can be obtained with a purity better than that which can be achieved by either of the known processes operated alone.
Two specific integrated processes are proposed. In each case there is a carbon sieve section and a zeolite sieve section each operating on psa cycles, the air feedstock being passed to a first of those sections and the oxygen-rich gas stream obtained thereby being passed as feedstock to the second section from which a high purity oxygen product is obtained. At the same time the overall preformance of the process is enhanced by the recycling as feedstock of an oxygen-rich gas stream from the second section to the first.
The carbon sieve section preferably comprises two beds each of which undergoes a cycle of feedstock admission and desorption, one bed being on the feed step of its cycle while the other bed is on the desorption step of its cycle and vice versa. The zeolite sieve section preferably comprises two beds each of which undergoes a cycle of feedstock admission, desorption and backfilling, the bed cycles being sequenced in the well-known manner so that one bed is on its feed step while the other bed is on its desorption and backfilling steps and vice versa.
In the preferred process dry, carbon dioxide free air is provided as feedstock to the carbon sieve section, as also are oxygen-rich gas streams obtained partly from the evacuation of a bed in the carbon sieve section and partly from the evacuation of a bed in the zeolite sieve section. With a two-bed carbon sieve section operating as indicated above, the evacuation of each bed in that section yields a gas stream of gradually increasing and then decreasing oxygen purity, the stream obtained during the middle part of the evacuation step being of about 80% oxygen purity and is fed to a first reservoir, while the streams obtained during the other parts of the evacuation step average about 50% purity and are fed to a second reservoir for eventual recycling as part of the feedstock to the carbon sieve section. The balance of the 80% oxygen stream is nitrogen and argon approximately in their atmospheric ratio, i.e. about 19.75 % nitrogen and 0.25% argon. This middle cut product from the carbon sieve section is used as the feedstock for the zeolite sieve section which, being capable of effecting virtually complete separation as between the oxygen and nitrogen, can provide a product stream with a proportion of oxygen as high as 99.7%, the balance being the small amount of argon passed from the carbon sieve section and which remains unadsorbed by passage through the zeolite sieve section. Thus it will be appreciated that the key to the high purity of the oxygen product obtainable with this integrated process resides not just in the ability of the carbon sieve section to provide an oxygen-enriched feedstock to the zeolite sieve section but in its ability to provide a feedstock which is depleted in argon, the one major constituent of atmospheric air which a zeolite sieve is incapable of separating from oxygen. Simple though this factor may appear, it is one which the prior art has singularly failed to recognise.
In the alternative process air is provided as feedstock to the zeolite sieve section as also is an oxygen-rich gas stream obtained during a feed step in the carbon sieve section. The zeolite sieve section gives a product containing approximately 90% oxygen with 5% nitrogen and 5% argon which is passed as feedstock to the carbon sieve section. As before the evacuation of each bed in the carbon sieve section yields a gas stream of gradually increasing and then decreasing oxygen purity. In this case the high purity stream obtained during the middle part of the evacuation step constitutes the product from the integrated process while the streams obtained during the other parts of the evacuation step are fed to a reservoir for eventual recycling as part of the feedstock for the carbon sieve section.
Once again the key to the high purity of the oxygen product obtainable with this process resides in the ability of the carbon sieve section to effect a separation as between oxygen and argon, which cannot be effected by the zeolite sieve section.
It is observed here that it has in the past been proposed to enrich a three or more component gas mixture with a desired component by passage through serially connected beds containing materials each of which selectively adsorbs one of the other components of the mixture. An example is U.S. Pat. No. 3,102,013 of C. W. Skarstrom. However, as will be more apparent from the ensuing description of preferred embodiments, the sequencing of the psa cycles performed at each section in the integrated processes of the present invention goes far beyond the simple serial feeding of gas through successive beds as proposed in the prior art.
The above-referenced Skarstrom patent states as one of its objects `to provide a method whereby oxygen/nitrogen-rich products may be recovered from atmospheric air without liquefaction or other expensive or complicated procedures`. Be that as it may, Skarstrom gives no practical example of a process for obtaining a high-purity oxygen product from an air feedstock, his examples being concerned primarily with the separation of hydrogen/methane/heavy ends and methyl alcohol/water vapour/nitrogen mixtures. More particularly Skarstrom fails to teach such essential features of the present invention as a zeolite molecular sieve section producing an oxygen rich gas stream during a feed step, a molecular sieve carbon section producing an oxygen rich gas stream by desorption of adsorbed components and effecting a separation as between oxygen and argon, and the recycling as feedstock of an oxygen-rich gas stream from the second section to the first in addition to the passage of the enriched product stream from the first section to the second.